Following my own concerns for natural fibers, I found an Organic Hemp Bag, the Large is 21" x 23" and just fits a tall 10 Gallon kettle (specifically, a snug fit over a 10 Gal Blichmann Boilermaker (G1)).
The proprietor, Abigail, says that, "Hemp is a unique fiber in that it gains strength when it is wet. Hemp is naturally resistant to bacteria. The weave of the hemp bags makes them the perfect filter for home brewing."
and this bag also carried at Seven Bridges Organic Brewing Supply: